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A plethora of materials, like foams, emulsions, gels, clays, etc, elude the ordinary distinction between fluid and solid. They can display both a viscoplastic fluid-like behaviour or an elastic solid-like one, depending on whether the applied load exceeds or not a certain critical value. Therefore, they are called complex fluids [1]. Unlike simple liquids, these materials are typically multiphase/multicomponent systems, as e.g. mixtures of fluids or suspensions of solid particles in a liquid. The complexity resides in the fact that the micro-/mesoscopic structure and dynamics determine the macroscopic mechanical response of the material. Modelling the rheology of complex fluids requires, then, an inherently multiscale approach, resulting in a formidable task. Commonly, the rheology of suspensions is primarily controlled by the volume fraction of solids. However, it has been recently shown [2] that adding small amounts of a liquid, immiscible with the suspending one, affects strikingly the mechanical response of the system, owing to the formation of par- ticle networks sustained by capillary forces. Such capillary suspensions may display elasto-visco-plastic behaviour, thus being useful for a number of technological applications. By means of lattice Boltzmann (LB) [3] simulations of binary fluid mixtures with suspended resolved solid particles [4], I will address a number of open questions, concerning: i) the dependence of the network structure upon the particle shape (spheres/ellipsoids) [5] and the particle-fluid affinity (measured by the contact angle), and, in turn, ii) the dependence of the rhelogical properties (namely the yield stress) on such structure. Analogous rheologi- cal properties (yielding point, shear-thinning behaviour) is shared by other complex fluids, like foams or dense emulsions. Here, the gas/liquid or liquid/liquid mixture is stabilised by surfactants, and the solid-like behaviour (below yield) is conferred to the material by the capability of storing elastic energy within the thin liquid films between neighbouring bubbles/droplets. I will present a lattice Boltzmann method for the simulation of such soft-glassy materials [6], based on a suitable ’on-lattice’ implementation of competing interactions, which proved to be a helpful tool to shed light on the flowing properties of confined foams and concentrated emulsions. In particular, I will focus on the role played by plastic rearrangements of neigh- bouring bubbles in determining the macroscopic non-local rheology, also in comparison with experiments and theoretical results [7, 8, 9].
[1] R. G. Larson, The structure and rheology of complex fluids. Oxford University Press (1999). [2] E. Koos and N. Willenbacher, Science 331, 897-900 (2011). [3] S. Succi, The lattice Boltzmann equation for fluid dynamics and beyond. Oxford University Press (2001). [4] F. Jansen and J. Harting, Phys. Rev. E 83, 046707 (2011); S. Frijters, F. Gu ̈nther and J. Harting, Soft Matter 8 (24), 6542-6556 (2012). [5] G.B.Davies,T.Kru ̈ger,P.V.Coveney,J.HartingandF.Bresme,Adv.Materials26,6715-6719(2014). [6] R. Benzi, S. Chibbaro and S. Succi, Phys. Rev. Lett. 102, 026002 (2009). [7] L. Bocquet, A. Colin and A. Ajdari, Phys. Rev. Lett. 103, 036001 (2009). [8] B. Dollet, A. Scagliarini and M. Sbragaglia, J. Fluid Mech. 766, 556-589 (2015). [9] A. Scagliarini, B. Dollet and M. Sbragaglia, Colloids Surf. A: Physicochem. Eng. Aspects 473, 133- 140 (2015). |